The `AssertWell\PHPUnitGlobalState\Functions` trait exposes three methods for dealing with functions:

1. `defineFunction(string $name, \Closure $func): self`
2. `redefineFunction(string $name, \Closure $func): self`
3. `deleteFunction(string $name): self`

This commit also adds additional documentation around runkit(7), as it's now used by both the `Constants` trait and `Functions`.

Fixes #11.

…with the Runkit trait in favor of more static methods on the Runkit support class